(a) Field of the Invention
This invention relates to silicon carbide used as a reinforcing material for other materials and more particularly relates to a previously unknown form of silicon carbide usable for this and other purposes, to a method for the manufacture of such novel silicon carbide, to a method for using same, and to the resulting novel reinforced products.
(b) Background of the Invention
There is presently an ongoing worldwide research effort to utilize silicon carbide whiskers and fibers as reinforcing agents. Presently known whiskers, usually single crystals having a high aspect ratio, are based on beta phase or cubic structure silicon carbide and as a result are not well suited for fabrication of ceramic composites which require processing temperatures above 1800.degree. C. due to the limited thermal stability of beta phase silicon carbide.
Nevertheless, attempts have been made to use such beta phase silicon carbide as reinforcing materials.
Examples of such prior art are discussed below.
U.S. Pat. No. 4,410,635, to Brennan et al., discloses discontinuous silicon carbide fiber reinforced ceramic composites which are formed by starting with the ceramic matrix material in a glassy state and converting it from a glassy state to a ceramic state after densification of the composite.
U.S. Pat. No. 4,399,231, to Prewo et al., discloses discontinuous silicon carbide fiber reinforced glass composites in which the silicon carbide fibers are laid up in substantially in-plane random direction orientation through utilization of a silicon carbide paper.
U.S. Pat. Nos. 4,387,080; 4,467,647; and 4,467,042, to Hatta et al., disclose the manufacture of flaky beta silicon carbide from an organic silicon polymer containing a metallic or non-metallic element such as Si, B, Ti, Fe, Al, Zr, Cr, and the like. The organic silicon polymer is molded into a thin sheet which is thereafter subjected to an anti-fusion treatment. This sheet is thereafter heated to a high temperature in an atmosphere of non-oxidative gas such as N.sub.2, H.sub.2, NH.sub.3, Ar, and CO. The thermal treatment is carried on at a temperature not exceeding 1800.degree. C. The resulting product is flaky beta silicon carbide. The infusible sheet of organic silicon polymer can be cut into small flaky pieces, each of which has a length and breath 10-100 times greater than the thickness thereof, and these pieces may be converted to flakes of beta silicon carbide. Alternatively, flaking may be effected after heat treatment of a larger sheet. Also disclosed is the application of thin sheets or flaky materials for dispersion of thermal stress in composite materials such as rubber, plastics, metals, and concretes. Flaky beta silicon carbide having a width and length in the range of 10-100 times greater than the thickness thereof is taught as resisting breakage in an extrusion molding machine. Such beta silicon carbide does not, however, have high temperature resistance.
U.S. Pat. No. 3,661,662, to Allen, discloses a process for making sheets of material in which flakes of silicon carbide or boron carbide are floated on a pool of liquid metal which is inert to the flakes and bonding the concentrated flakes together on the surface of the pool to form a sheet thereof which is withdrawn from the surface of the pool. The bonding material is an organic resin.
Alpha silicon carbide including alpha silicon carbide of a hexagonal crystal structure is known. Such materials have not, however, been particularly suitable for use as reinforcing materials because in the prior art it was not possible to, or at least not practical to, consistently make silicon carbide having a structure which was sufficiently pure and flawless to act as a good reinforcing material.
Individual large, typically between 0.1 and 3 cm, and usually intergrown hexagonal crystals sometimes spontaneously appear during synthesis of silicon carbide by Acheson electrofurnacing. Such crystals are, however generally too large and too few relative to total silicon carbide prepared in the furnace, to be used as reinforcing materials. Even if such crystals were individually collected and crushed to smaller sizes, the result would not be a good reinforcing material since such crushing operations results in a large number of flaws in the particles of the finished, crushed materials and also results in particles having an undesirable shape and size.
U.S. Pat. No. 3,962,406, to Knippenberg et al., discloses an inefficient method of manufacturing silicon carbide crystals in which a core of silicon dioxide is embedded in a mass of granular silicon carbide, or materials which form silicon carbide. Heating this mass to a temperature at which silicon dioxide volatilizes, i.e. above about 1500.degree. C., results in formation of a cavity surrounded by silicon carbide. After formation of the cavity, heating is continued at a temperature above about 2500.degree. C., at which silicon carbide crystals of plate shape are formed on the walls of the cavity.
Another patent disclosing the manufacture of hexagonal silicon carbide crystals is U.S. Pat. No. Re. 26,941 to Lowe. This patent describes the preparation of large, ultrapure crystals by a slow and arduous vapor deposition process for electronic purposes for rectifiers and transistors. The crystals are up to 0.75 inches across with thicknesses of from 1 to 100 mils (25 to 2540 microns), see e.g. column 5, lines 59-61. Such materials are generally too large for most reinforcing applications.
American Matrix, Inc., formerly Phoenix International, of Knoxville, Tenn., has recently announced the availability of particles of alpha silicon carbide for reinforcement of composite materials. The manner in which this material is made, however, has not been published. These particles under a microscope appear to be the result of crushing large particles. The product appears to be a mixture of various structures of various shapes, e.g. needles, powder, and fragments, including some hexagonal crystal material. The particles hava a large number of faults and analysis indicates a low purity.
In summary, there are known to those skilled in the art many routes for the preparation of alpha or beta type silicon carbide from a variety of raw materials. However, there is no teaching or suggestion that thin single crystal, hexagonally shaped platelets of the more stable alpha type silicon carbide could intentionally be formed, nor is there any teaching as to how this may be accomplished on demand, nor that such platelets would have any unexpected utility.
Additionally, there is no teaching or suggestion of a porous silicon carbide matrix of small hexagonal crystal structure, i.e. where the base faces are separated by from 0.5 to 20 microns, nor any teaching or suggestion of a use for such a matrix.